In certain laser imaging systems a laser sensitive media is mounted onto the surface of an imaging cylinder and an image is imparted onto the media using a focused write laser of relatively high power. FIG. 1 shows a prior art external drum imaging system 1 having an imaging head 2 directing a laser beam or beams 18 toward a media 4. Media 4 is rotated on a drum 5 while imaging head 2 is translated along the drum by a leadscrew 6, thus scanning or writing a series of bands or a helical pattern around the drum.
In other laser imaging systems a media is held on a flatbed platen and relative motion is generated in two dimensions between the media and the imaging beam. Such imaging systems are used in devices for imaging many different kinds of media including lithographic plates, flexographic plates, and screens for screen printing, as well as layers for flat panel displays, printed circuit boards and the like. Such imaging systems could be incorporated directly on a printing press for imaging plates in-situ. Such systems are well known in the art and will not be discussed further in this application.
During imaging, the interaction of the laser and the media causes a physical and/or chemical change to the imaged areas of the media. In the process of imaging, matter may be expelled from the laser sensitive media. The expulsion of matter from the media is referred to as ablation. The matter expelled may consist of solids, liquids, gases, and plasma. The matter is sometimes called smoke or particulate debris. Ablative media are imaged by dislodging or evaporating material from a layer of the media to form an image. While ablative media by nature produce ablation debris, media traditionally regarded as non-ablative also produce fumes and/or particle debris, particularly when imaged by high power lasers, albeit in smaller quantities. A certain amount of ablated debris remains on the surface of the imaged media after imaging.
When loading and unloading media onto the media-bearing surface, a roller is commonly employed to guide the media on or off of the surface. The roller may also help to hold the media against the surface. Typically, the roller is covered with a soft, relatively smooth material, which will not damage the laser-sensitive emulsion of the media. FIG. 2 shows an existing external drum imaging system during the media loading process. While one end of the media 4 is secured to drum 5 with a clamp 10, a roller 15 is brought into contact with the surface of the media 4. Roller 15 forces media 4 against the drum's surface while the drum rotates 12 to load the media onto the drum's surface. The other end of the media can then be clamped with a second set of clamps 20 to fully secure the media onto the drum, and then the roller 15 can be released. The same system can be employed in unloading the media, by reversing the direction of rotation 17 of the drum. Roller 15 is brought into contact with the now laser-imaged media 4 near the clamps 20 on one end of the media, and then the clamps 20 are released. The drum 5 is rotated 17 in the opposite direction from the loading process until the roller is near the other set of clamps 10. The roller 15 is then released, along with the clamps 10, so that the plate can be removed.
If there is remnant debris on the media's imaged surface, a portion of this debris is transferred onto roller 15 during the unloading process. Additionally, after repeated loading and unloading cycles, debris tends to further accumulate on roller 15. Debris on roller 15 can transfer onto the media's surface during the load cycle. This debris can then mask the laser's exposure of the media's surface, thus causing imaging artefacts.
One way to address the problem of debris accumulation on the roller is by periodic cleaning of the roller. This can be done either by the machine operator or by some sort of automated roller cleaner. Having the machine operator clean the roller is perceived as an annoyance, and results in machine down time. Thus, there is a need for increasing the interval between cleanings or to eliminate them altogether. One example of an automated roller cleaner is in the Trendsetter™ Spectrum™ device sold by Creo Inc of Burnaby, British Columbia, Canada. This machine, which images proofing and other media, is highly sensitive to regular dust normally found in an office environment. The roller cleaner comprises a separate sticky roller, with a consumable, tear-off sticky coating. The sticky roller contacts and rolls against the media-contacting roller, removing a good portion of the dust and debris. This solution requires periodic replacing of the sticky material by the machine operator. The sticky material is a consumable cost to the machine owner. Also, this solution requires that the architecture of the machine's frame be designed to specifically accommodate this sticky roller. This may involve a significant up-front cost. It may not be feasible to upgrade existing equipment to include such a sticky roller. There is thus a need for a solution that does not require substantial machine alterations, and preferably involves less maintenance.
The cleaning interval for rollers depends in part on the material from which the rollers are made. The cleaning interval can be increased by making the surface of the rollers from a properly selected material. However, the surface of the media being imaged is coated in a laser-sensitive emulsion. This emulsion is often sensitive to having anything contact it. Therefore the material of the roller as well as the pressure the roller exerts on the media are important design considerations. This poses an additional complication in that the roller material should not damage the emulsion surface, in addition to not being prone to debris accumulation.